When you are having these issues, could you check the network tab in the developer tools (can either access from the menu in your browser, or normally pressing F12 will open them), and see if there is a request to sync that is returning a 502 error?
This seems like the most likely cause if it is then working on master. because that will have a higher tolerance for timeouts.
We are making some architectural changes to Studio in the coming months that will hopefully reduce the incidence of these kinds of issues, hopefully this will help!
Thank you very much for your answer, I will keep it in mind. I am also having problems in Kolibri when I want to import massive users, can I open another thread?